Windows Workflow Foundation (WF) is a Microsoft

Microsoft

Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 technology that provides an API

Application programming interface

An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

, an in-process workflow

Workflow

A workflow consists of a sequence of connected steps. It is a depiction of a sequence of operations, declared as work of a person, a group of persons, an organization of staff, or one or more simple or complex mechanisms. Workflow may be seen as any abstraction of real work...

 engine, and a rehostable designer to implement long-running processes as workflows within .NET

.NET Framework

The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...

 applications. The current version of WF was released as part of the .NET Framework

.NET Framework

The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...

 version 4 and is referred to as (WF4) .
A workflow, as defined here, is a series of distinct programming steps or phases. Each step is modeled in WF as an Activity.